An anti-lock brake system of this type is known already from German published patent application 37 31 603 which comprises a master cylinder to which the wheel brakes are connected via inlet valves. When actuated, outlet valves which are closed in their inactive position establish a pressure fluid connection from the wheel brakes to a pressure compensating reservoir. By means of a hydraulic pump, the pressure fluid is supplied from the supply reservoir back into the brake system, namely into the master cylinder, in consequence whereof simultaneously the working pistons of the master cylinder and hence the brake pedal as well are reset and positioned.
The master cylinder of this known brake system is equipped with travel sensors which permit to switch the hydraulic pump on and off in dependence on the forward stroke of the pistons. The master cylinder piston will be reset by way of switching on of the pump and development of a corresponding auxiliary pressure until the travel sensor responds and switches the pump off. This way the pistons and hence the brake pedal are positioned by virtue of the travel sensor in conjunction with the activated hydraulic pump.
Major difficulties are involved in practical operation to devise the adjustment of the pistons' position and/or the switching on and off of the pump in dependence on the actual pressure fluid requirement in such a manner that under various conditions and tolerances exactly the right quantity of pressure fluid is always delivered. A too small rate of delivery or a too late activation of the pump must be avoided by any means for safety reasons, while a sudden and excessive pedal resetting action is very unpleasant for the driver. Moreover, a too frequent operation of a hydraulic pump results in the occurrence of undesirable noise.